Method and apparatus for scheduling communication using a star switching fabric

ABSTRACT

In one embodiment, a scheduler for use with a star switching fabric includes a scheduling star switching fabric operable to receive a plurality of packets each associated with one of a plurality of wavelengths and a plurality of selecting elements associated with the scheduling star switching fabric. Each of the plurality of selecting elements is operable to contribute to selectively passing packets from the scheduling star switching fabric for receipt by a transmission star switching fabric. Packets received at the transmission star switching fabric over a given time period comprise a more uniform load distribution than packets received at an input to the scheduler over the same period of time.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of communication systems, andmore particularly to an apparatus and method for schedulingcommunication through a star switching fabric.

BACKGROUND

As optical systems continue to increase the volume and speed ofinformation communicated, the need for methods and apparatus operable tofacilitate high speed optical signal processing also escalates. Routerand switch cores performing optical switching generally implementschedulers to assist in avoiding contention for common system resources.In prior approaches, there has generally been a tension between thecomplexity of the scheduler used and the delay experienced in theswitching fabric. More complex schedulers generally require significantsystem resources and can be difficult to implement. Trivial schedulers,while simple to implement, have generally resulted in unsatisfactoryswitching delays.

Overview of Various Example Embodiments

The present invention recognizes a need for a method and apparatusoperable to efficiently and effectively facilitate scheduling ofcommunication through a star switching fabric. In one embodiment, ascheduler for use with a star switching fabric comprises a schedulingstar switching fabric operable to receive a plurality of packets eachassociated with one of a plurality of wavelengths, and a plurality ofselecting elements associated with the scheduling star switching fabric.Each of the plurality of selecting elements is operable to contribute toselectively passing packets from the scheduling star switching fabricfor receipt by a transmission star switching fabric. Packets received atthe transmission star switching fabric over a given time period comprisea more uniform load distribution than packets received at an input tothe scheduler over the same period of time.

In a method embodiment, a method of scheduling operation of a starswitching fabric comprises receiving at a scheduler a plurality ofpackets each having a wavelength and communicating from a schedulingstar switching fabric of the scheduler a plurality of substantiallysimilar sets of the plurality of packets. The method further comprisesselectively passing packets having selected wavelengths from thescheduling star switching fabric for receipt by a transmission starswitching fabric. Packets received at the transmission star switchingfabric over a given time period comprise a more uniform loaddistribution than packets received at an input to the scheduler over thesame time period.

Depending on the specific features implemented, particular embodimentsmay exhibit some, none, or all of the following technical advantages.One embodiment provides a way to schedule communication of opticalsignals through a star switching fabric using a simple schedulingalgorithm while maintaining good throughput. Other technical advantagesare readily apparent to one of skill in the art from the attachedfigures, description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and forfurther features and advantages thereof, reference is now made to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a block diagram illustrating an exemplary communication systemimplementing aspects of the present invention;

FIG. 2 is a block diagram of one example embodiment of an opticalimplementing aspects of the present invention;

FIG. 3 is a block diagram of another example embodiment of an opticalimplementing aspects of the present invention;

FIGS. 4 a–4 b are block diagrams illustrating example star switch fabricarchitectures;

FIGS. 5 a–5 d are block diagrams illustrating example schedulingmechanisms for use with star switching fabrics, including thosedescribed herein;

FIGS. 6 a–6 d are block diagrams illustrating additional examplescheduling mechanisms for use with star switching fabrics, includingthose described herein;

FIG. 7 is a block diagram illustrating an example embodiment of acontinuum optical source for use with a star switching fabric, includingthose described herein;

FIGS. 8 a–8 b are block diagrams illustrating example mechanisms usefulin increasing the speed of optical routers including those describedherein;

FIGS. 9 a–9 c are block diagrams illustrating additional examplemechanisms useful in increasing the speed of optical routers includingthose described herein;

FIG. 10 is a flow chart showing one example of a method of routingoptical signals using a star switching fabric;

FIG. 11 is a flow chart showing one example of a method of schedulingcommunications through a star switching fabric;

FIG. 12 is a flow chart illustrating one example of a method ofenhancing the effective switching speed of an optical router by reducingthe duration of packets communicated through a star switching fabric ofthe router;

FIG. 13 is a flow chart showing one example of a method of enhancing theeffective switching speed of an optical router by aggregating packetsbound for a common output communication path;

FIG. 14 is a flow chart showing one example of a method of enhancing theeffective switching speed of an optical router using a star switchingfabric by providing express lanes that bypass line cards performingelectronic signal processing of some of the optical signals received;

FIG. 15 is a flow chart showing one example of a method for enhancingthe effective switching speed of an optical router using a starswitching fabric by assigning a plurality of tunable filters to eachoutput link from the router; and

FIG. 16 is a flow chart showing one example of a method of enhancing theeffective switching speed of an optical router using a star switchingfabric by assigning a plurality of tunable transmitters to an input linkto the optical router.

DETAILED DESCRIPTION OF VARIOUS EXAMPLE EMBODIMENTS

FIG. 1 is a block diagram illustrating an exemplary communication system10 operable to facilitate communication of optical signals. In thisexample, system 10 includes a router 12 coupled to a plurality ofnetwork elements 20 a–20 n. Router 12 facilitates directing opticalcommunication signals between various elements within and/or coupled tosystem 10. Throughout this document, the term “coupled” denotes anydirect or indirect communication between two or more elements said to be“coupled” to one another. Elements coupled to one another may, but neednot, be physically connected to one another. Additional elements may ormay not reside between two elements said to be “coupled” to one another.

As used throughout this document, the term “router” refers to anyhardware, firmware, software, or combination thereof operable to receivesignals from various sources and to direct signals received toward oneor more destinations depending at least in part on an identifierassociated with the signal and its destination.

In one particular embodiment, signals received by router 12 comprisepackets. As used throughout this document, the term “packet” refers tosignals having fixed or variable size, each comprising an identifierassociated with a destination network element. While some of the packetsmay comprise traffic terminating at router 12, at least some of thepackets contain identifiers identifying destination elements external torouter 12. The packets could comprise, for example Internet Protocol(IP) packets or a Transmission Control Protocol (TCP) packets includingan address identifying a destination network element. As anotherexample, each incoming optical signal could comprise a Multi-ProtocolLabel Switching (MPLS) packet or Generalized Multi-Protocol LabelSwitching (GMPLS) packet comprising a tag identifying a destinationnetwork element.

In some cases, the “destination network element” may comprise a nodewithin or coupled to system 10, but external to router 12, to whichinformation in the optical signal is ultimately destined. In othercases, the “destination network element” may comprise a node external torouter 12 in a communication path between router 12 and an element towhich the information is ultimately destined. In that case, the“destination network element” comprises an intermediate network elementfacilitating further routing of the information to the ultimatedestination network element. In still other cases, router 12 maycomprise the destination element.

Network elements 20 a–20 n communicate optical signals over system 10.Network elements 20 may comprise any hardware, software, firmware, orcombination thereof operable to transmit and/or receive information viacommunication system 10. Router 12 communicates with network elements 20via communication links 22 a–22 n. Communication links 22 may comprise,for example, optical fibers. Communication links 22 a–22 n could,however, comprise any land based or space based communication medium, orcombination of such media operable to communicate one or more opticalsignals.

Network elements 20 can couple directly to communication links 22, ormay couple to communication links 22 through one or more networks 24.Each of networks 24 could comprise, for example, a data network, apublic switched telephone network (PSTN), an integrated services digitalnetwork (ISDN), a local area network (LAN), a wide area network (WAN),or other communication system or combination of communication systems atone or more locations. Networks 24 may comprise wireless networks,wireline networks, or combinations of wireless and wireline networks.Network elements 20 and/or router 12 can reside with networks 24 orexternally to those networks.

In this particular example, router 12 comprises a plurality of linecards 30 a–30 n. As used throughout this document, the term “line card”can include any hardware, software, firmware, or combination thereofoperable to receive incoming optical signals from communication links 22and to convert at least a portion of at least some of the incomingoptical signals to electrical signals to facilitate electronic decisionmaking with respect to those signals. In the illustrated embodiment,each line card 30 is associated with an optical transmitter operable togenerate, based at least in part on the electrical signals received, anoptical router signal for transmission within router 12. The opticaltransmitters may comprise, for example, laser diodes, light emittingdiodes, or other light emitting sources.

Line cards 30 may reside in one or more physically separate locations.In this particular example, a first plurality of line cards 30 a–30 mreside in a first rack 32, while a second plurality of line cards 30m+1–30 n reside in a second rack 34. As one specific example, first rack32 and second rack 34 may each hold sixteen line cards 30. Additional orfewer numbers of line cards and numbers of racks could be used withoutdeparting from the scope of the invention.

In this example, racks 32 and 34 are physically separated from oneanother. In one embodiment, racks 32 and 34 may be separated by adistance where communication speed considerations make it desirable toimplement optical communication between racks 32 and 34. Line cards 30in racks 32 and 34 advantageously use optical communication links 42a–42 n to facilitate high speed communication. In this particularexample, optical communication links 42 a–42 n interconnect through anoptical switching fabric 40.

Switching fabric 40 comprises hardware, software, firmware, orcombinations thereof operable to facilitate directing optical routersignals between line cards 30 and/or express channels (not explicitlyshown in this figure), which bypass line cards 30. In a particularexample, switching fabric 40 comprise a star switching fabric.Throughout this document, the term “star switching fabric” refers to adevice and/or functionality operable to receive a plurality of inputoptical signals from a plurality of sources and to communicate asubstantially similar set of at least some of the input optical signalsto each of a plurality of destinations. In one particular embodiment,star switching fabric 40 resides within one of racks 32 or 34.

In the example illustrated in FIG. 1, switching fabric 40 could comprisea star switching fabric operable to receive a plurality of input opticalrouter signals from plurality of line cards 30 and to communicatesubstantially similar sets each comprising at least some of the inputoptical router signals back to at least some of the plurality of linecards 30 and/or express channels bypassing line cards 30. Fused fibercouplers, waveguide star couplers, arrayed waveguide gratings, powersplitters, wavelength division multiplexers, cascaded 2×2 couplers, n×ncouplers and cascades of these couplers are just a few examples ofdevices that could form star switching fabric 40.

In a particular embodiment, switching fabric 40 advantageouslyinterconnects line cards 30 residing within different racks 32 and 34,and facilitates communicating optical router signals between line cards30 without requiring electrical-to-optical or optical-to-electricalsignal conversions within switching fabric 40. This design can increasethe speed of the router, and could also reduce the physical size, powerdissipation, and cost of the router. In one particular embodiment,switching fabric 40 could occupy less than one third of the space ofrack 32 or 34, leaving substantial room for additional line cards andother processing elements.

In one example embodiment, router 12 includes a plurality of tunablefilters. A tunable filter can comprise any hardware, software, and/orfirmware operable to selectively substantially communicate one or morewavelengths of light while substantially rejecting other wavelengths oflight. In this example, each tunable filter is associated with one ofline cards 30 or with an express channel.

Each tunable filter is operable to receive a plurality of opticalsignals and to select one or more signals for processing by tuning to awavelength associated with the selected signals. The use of tunablefilters in router 12 advantageously facilitates efficient multicastand/or broadcast operation simply by tuning multiple filters, eachassociated with a separate line-card or express channel, to a commonwavelength.

In operation, router 12 receives a plurality of input optical signalsfrom communication links 22. One or more optical links can carry signalsat wavelengths designated as express channels within router 12. Expresschannels route directly through switching fabric 40 from inputs ofrouter 12 to outputs of router 12, bypassing line cards 30.

With respect to non-bypass traffic, line cards 30 receive at least someof the input optical signals and convert all or a portion of thosesignals to an electronic format to facilitate electronic decision makingprocessing. As one particular example, one or more line cards 30 receivepackets and convert at least a destination identifier portion of thepacket into an electronic format. Line cards 30 then use the electronicdestination identifier information to assist in directing the packet toa destination network element.

Optical transmitters associated with line cards 30 generate inputoptical router signals based at least in part on processing of theelectronic signals. Router 12 communicates the input optical routersignals and any bypass traffic to switching fabric 40, where a pluralityof input optical router signals and any bypass traffic are combined toform an output optical router signal. The output optical router signalcomprises information from some or all of the plurality of input opticalrouter signals and/or express channel signals.

Switching fabric 40 facilitates communicating the output optical routersignal to at least some of a plurality of tunable filters, eachassociated with an output link from router 12. Tunable filters receivethe output optical router signal and tune to a selected wavelengthassociated with a portion of the output optical router signal destinedfor a line card 30 associated with that filter or an express channeloutput link associated with that filter. The selected portion of theoutput optical router signal can carry the packet bound for thedestination network element.

Where a line card 30 is associated with the tunable filter, the linecard facilitates communication of the received packet from theassociated filter to the destination network element. This may include,for example, passing the packet in optical form to an outputcommunication link, or converting the packet to an electrical format forfurther processing within router 12. Router 12 may also performwavelength conversion prior to passing the signals toward thedestination network element.

FIG. 2 is a block diagram of one particular embodiment of router 112. Inthis example, router 112 includes a plurality of wavelength divisionmultiplexer/demultiplexers (WDM) 110 a–110 n. Each WDM is associatedwith one or more optical links 122 carrying wavelength divisionmultiplexed optical signals. Wavelength divisionmultiplexers/demultiplexers 110 receive incoming WDM signals fromoptical links 122 and separate the incoming signal into a plurality ofchannels λ₁–λ_(n) for processing within line cards 130. On the outputside, wavelength division multiplexers/demultiplexers 110 combine aplurality of signals into one or more multiple wavelength outputsignals.

In this particular example, incoming signals received at links 122 alsoinclude one or more express channels λ_(Ex), which traverse router 112over bypass links 155 without being processed by line cards 130. Expresschannels λ_(Ex) are communicated directly to switching fabric 140without any optical-to-electrical conversion. Implementing expresschannels can provide significant advantages in avoiding unnecessaryprocessing of particular groups of optical signals. Although thisexample shows just one express link, any number of express links couldbe provided. Traffic entering router 12 can be divided between processedtraffic and express traffic, for example, by designating particularwavelengths in WDM signals 122 accordingly.

In the illustrated embodiment, router 112 comprises a plurality of racks132 a–132 n of line cards 130 each coupled to a switching fabric 140. Inthis example racks 132 a and 132 n are physically separated from oneanother and switching fabric 140 serves as an all-optical interconnectbetween line cards 130 in racks 132 a and 132 n. In other embodiments,all line cards 130 could reside locally to one another, for example, ina single rack.

In the illustrated example, each line card 130 includes a processor 136.Alternatively, some of all of line cards 130 could share centralprocessing resources accessible to line cards 130. In any case,processor or processors 136 operate to convert at least a portion of aninput optical signal 128 arriving from one of communication links 22 toan electrical format. For example, input optical signal 128 may comprisea packet having a destination identifier, such as a TCP address, an IPaddress or, an MPLS or GMPLS tag. Processor 136 operates to convert atleast the destination identifier portion of the packet to an electricalformat to facilitate electronic decision making functions with respectto that packet.

In this example, each line card 130 comprises a memory 138. Memory 138may comprise any hardware, software, and/or firmware operable tofacilitate storage and/or retrieval of electronic information. Althoughin this example memory 138 is shown as residing entirely within linecard 130, all or a portion of memory 138 could alternatively reside atanother location remote from but accessible to line card 130.

Each memory 138 stores a look-up table 144 operable to facilitateelectronic decision making to result in communicating incoming opticalsignals 128 from router 112 toward destination network elements residingexternally to router 112. Look-up table 144 may comprise any datastructure, compilation, or other arrangement of information facilitatinggeneration of instructions based at least in part on informationcontained in a signal to be routed. As one particular example, using anidentifier of the destination element from a received packet, processor136 may index look-up table 144 to obtain instructions on directing thepacket through router 112 and toward the destination element. Look-uptable 144 can, for example, facilitate TCP/IP routing based on anaddress associated with the destination element. Alternatively, look-uptable 144 can facilitate label switching based on an MPLS or GMPLSrouting protocol.

In some cases, router 112 may comprise an edge router facilitatingcommunication of packet traffic received in one format through asubnetwork operating with another format. For example, router 112 couldreceive IP or TCP packets from an IP network and convert those packetsto an MPLS or GMPLS format for transmission through a label switchingportion of a network. In that case, the packets traversing switch fabric140 would comprise MPLS or GMPLS packets.

Each line card 130 a–130 n further comprises an optical transmitter 146a–146 n operable to receive an electronic signal 129 a–129 n and togenerate an input optical router signal 152 a–152 n, respectively, basedat least in part on the received electronic signal 129 a–129 n. Eachoptical transmitter may comprise, for example, a laser diode, althoughany optical transmitter could be used without departing from the scopeof the invention. Optical transmitters 146 may comprise directlymodulated or externally modulated lasers. Alternatively, one or more ofoptical transmitters 146 may comprise lasers having integratedmodulators, such as electro-absorbtion modulators.

In one particular embodiment, each optical transmitter 146 comprises afixed wavelength laser. Throughout this document, the term “fixedwavelength laser” denotes a laser operable to generate optical signalsat approximately one predetermined wavelength or range of wavelengths,and which does not during operation perform selective adjustment of theoutput wavelength. Lasers whose output wavelength varies duringoperation due to, for example, fluctuations in environmental conditionsare not intended to be excluded from the definition of a “fixedwavelength” laser. Moreover, tunable lasers operated withoutintentionally selectively varying the output wavelength of the laserduring operation are intended to be within the definition of a “fixedwavelength” laser.

Although some embodiments of the invention implement tunable lasers,using fixed wavelength lasers 146 provides an advantage of reducing costand complexity of router 112 compared to solutions requiring tunablelasers. In addition, one aspect of the invention recognizes that usingfixed wavelength lasers, each transmitting at a different wavelength,reduces or eliminates collisions in the switching fabric.

In this example, each optical link 128 is associated with a tunablefilter. In the illustrated embodiment, each of line cards 130 a–130 nincludes a tunable filter 148 a–148 n, respectively. Each expresschannel 127 also includes a tunable filter 148 ex 1–148 exn. Tunablefilters 148 may each comprise, for example, a tunable optical filteroperable to selectively communicate particular optical router signals152 from output optical router signal 154. As one example, tunablefilters 148 could each comprise a Fabry Perot interferometric device. Ina particular embodiment, the filter could comprise amicro-electromechanical switch (MEMS) device capable of tuning at speedsfaster than once each one hundred nanoseconds.

Although many other tunable filter designs could be implemented withoutdeparting from the scope of this disclosure, the following provides abrief description of one such device.

A Fabry Perot interferometric micro electromechanical switching (MEMS)device typically implements a stationary mirror structure and a moveablemirror structure, which form between them an optical cavity having adepth that can be selectively altered by applying a force to themoveable mirror structure. In one particular novel design, the moveablemirror structure can be supported by actuators surrounding the moveablemirror structure.

The actuators can comprise, for example, a stationary conductor and amoveable conductor, which form between them an electrode gap. A voltagedifference applied between the two conductors creates an electrostaticforce tending to move the moveable conductor toward the stationaryconductor.

The actuators can be placed in symmetric locations around the moveablemirror and coupled to the moveable mirror. Locating the actuators aroundthe mirror facilitates independent selection of the nominal opticalcavity depth and the electrode gap depth. Thus, this design facilitatesoptimizing both the optical characteristics of the interferometerthrough selection of the optical cavity depth, and separate optimizationof the electrical characteristics of the device through independentselection of the electrode gap depth. Moreover, by forming theinterferometer and actuators in this manner, the dimensions of themoveable conductor can be optimized to provide high speed and low drivevoltage.

In some embodiments, the moveable mirror assembly of the interferometercan be supported by a frame that substantially surrounds and/or coversthe moveable mirror. The frame and location of the actuators help toavoid deformation of the moveable mirror structure during actuation,resulting in better optical characteristics for the device. Althoughdetails of one particular tunable filter have been described here, othertunable filter designs could be used. Other MEMs designs, lithiumniobate tunable filters, and liquid crystal tunable filters provide afew examples.

Line cards 130 can also include a converter 149 operable to convert therecognized portion 152 of output optical router signal 154 into anelectrical signal 129 for further processing within router 112.

Router 112 includes a control network 160 operable to communicatecontrol signals 162 to facilitate selection of a communication paththrough router 112 and on to the destination element. In one embodiment,control signals 162 direct tunable filters 148 to tune to a specifiedwavelength or range of wavelengths to facilitate selection of anappropriate optical router signal 152 from multiple wavelength outputoptical router signal 154. As a particular example, control network 160could comprise an Ethernet. Although other control networkconfigurations could be used without departing from the scope of theinvention, an Ethernet provides an advantage of efficient and economicaloperation at speeds sufficient to control and reset filters 148 betweenreceipt of sequential optical router signals.

In an alternative embodiment, control network 160 could comprise aplurality of control lasers each operable to generate and communicate tofilters 148 an optical control signal 162 at, for example a designatedcontrol frequency. In this embodiment optical control signals arecommunicated via switching fabric 140. Router 112 may, for example,communicate control signals to filters 148 prior to communicatingoptical router signals to filters 148. In that way, filters 148 can beprovisioned to accept selected optical router signals 152 depending onthe state of an optical control signal 162.

Router 112 may include a scheduler 164 coupled to control network 160.Scheduler 164 can operate to provide scheduling functionality to avoidor reduce contention in transmission of control signals 162 to filters148. FIGS. 5 a–5 d discussed below provide details of example schedulingmechanisms useful with any star switching fabric, including the designdiscussed herein with respect to FIGS. 2 and 3.

Router 112 interconnects line cards 130 using switching fabric 140including communication links 143 and 145. Communication links 143couple lasers 146 to switching fabric 140, while communication links 145couple filters 148 to switching fabric 140. In this example,communication links 143 and 145 comprise single mode fibers.

In operation, wavelength division multiplexer/demultiplexers 110 receiveone or more multiple wavelength signals 122 and separate input signals128 a–128 n including express channels 127 from one another. Expresschannels 127 a–127 n are directed to switching fabric 140 withoutperforming optical-to-electrical conversions on those signals.

Processor(s) 136 associated with line cards 130 receive input opticalsignals 128 a–128 n and converts at least a portion of each signal to anelectronic format. In one embodiment, processor(s) 136 can operate toconvert to an electronic form the entire contents including the headerand payload portions of incoming optical signal 128. Processor(s) 136apply at least a destination identifier portion of the electronic signal129 to look-up table 138 to determine communication instructions for thesignal. Optical transmitter 146 can then form an optical router signal152 by transforming electronic information into optical router signal152.

In another embodiment, processor(s) 136 may convert only a headerportion of input optical signal 128 to electronic form leaving thepayload portion in optical form. In that case, processor(s) 136 mayperform electronic processing on the header to determine routing of thesignal, and then pass the header or a modified version thereof tooptical transmitter 146. In that embodiment, optical transmitter 146produces an optical header, which is then combined with the opticalpayload portion of the signal to form an optical router signal fortransmission through switching fabric 140. In that embodiment, theportion of the input optical signal that is not converted to anelectronic format can be passed through a delay element, such as abuffer or a delay line, to facilitate delay while the identifier portionof the packet is electronically processed.

Each optical transmitter 146 communicates to switching fabric 140 anoptical router signal 152 at a particular wavelength. Where opticaltransmitters 146 comprise fixed wavelength lasers, each opticaltransmitters 146 transmits its optical router signal 152 at apredetermined specified wavelength associated with that particulartransmitter 146, which is different from wavelengths transmitted fromother transmitters 146. Where optical transmitters 146 comprise tunablelasers, each laser communicates its optical router signal 152 at awavelength determined by a control signal from, for example, processor136.

In this particular embodiment, each processor 136 determines a controlsignal 162 based at least in part on applying a destination identifierto the look-up table 144 associated with that line card 130. In someembodiments control signal 162 may identify an output communication link128 coupling to the destination network element. In other cases, controlsignal 162 may identify a filter 148 associated with the identifiedoutput link 128. Router 112 communicates control signals 162 via controlcircuitry 160 to tunable lasers 146 and/or tunable filters 148 toselectively enable communication paths through router 112.

Transmitters 146 each communicate an optical router signal 152 toswitching fabric 140. In this particular example, switching fabric 140comprises a star coupler switching fabric. Star coupler switching fabric140 receives a plurality of optical router signals 152 and may alsoreceive one or more express channels 127 each having substantiallydifferent wavelengths. Switching fabric 140 combines information from atleast some of the optical router signals 152 and/or at least some of theexpress channels 127 into an output optical router signal 154. Eachoutput optical router signal 154 comprises a substantially similar setof optical router signals 152 and/or express channels 127. Starswitching fabric communicates optical router signal 154 to some or allof filters 148.

In a particular embodiment, transmitters 146 comprise fixed wavelengthlasers while filters 148 comprise tunable filters. This embodimentprovides an advantage of minimizing cost by implementing low costtunable filters as compared to relatively higher cost tunable lasers. Inaddition, implementing tunable filters readily facilitates multicastand/or broadcast operation simply by provisioning the tunable filters toreceive a plurality of the optical router signals communicated fromswitching fabric 140.

In this example, router 112 communicates control signals 162 toscheduler 164 and/or to a tunable filter 148 associated with acommunication path leading to the destination network element. Filters148 receive control signals 162 and selectively tune to receiveparticular wavelengths as directed by control signals 162. In thismanner, tunable filters 148 selectively receive only the portion ofoutput optical router signal 154 communicated from switching fabric 140that is intended for further transmission toward the destinationelement.

In an alternative embodiment, transmitters 146 may comprise tunableoptical lasers. In that embodiment, lasers 146 may receive controlsignals 162 and communicate optical router signals 152 to switchingfabric 140 at selected wavelengths predetermined to match wavelengths offilters 148 associated with communication paths leading to thedestination network elements.

Filters 148 receive specified portions of output optical router signal154 corresponding to the packet desired for transmission to thedestination network element. In one embodiment, each filter 148comprises an optical filter operable to communicate only optical routersignals having a specified wavelength. In a particular embodiment, thereceived optical router signal can be communicated without furtherprocessing in router 112 to the destination network element. In anotherembodiment, each line card 130 may also include a converter 149 operableto convert an optical router signal received from an associated filter148 to an electronic format for further processing within router 112before conversion back to an optical format to be communicated towardthe destination network element.

FIG. 3 is a block diagram illustrating another embodiment of a router212. Router 212 is similar in structure and function to router 112 shownin FIG. 2, except that in this case, tunable filters 248 reside remotelyfrom line cards 230 and in close proximity to or integrally withswitching fabric 240.

Router 212 includes a plurality of line cards 230 each associated withan optical transmitter 246 and a tunable filter 248. Each line card 230is coupled to a switching fabric 240 via communication links 243 and245. Switching fabric 240 operates to receive a plurality of inputoptical router signals 252 a–252 n from optical transmitters 246 a–246 nand one or more express channel signals 227 and to generate an outputoptical router signal 254 comprising information from at least some ofthe input optical router signals 252 a–252 n and/or express channelsignals 227.

In one particular embodiment, optical transmitters 246 comprise fixedwavelength transmitters each operable to generate a particularwavelength signal. In this embodiment, filters 248 each comprise atunable optical filter operable to receive multiple signals each havingdifferent wavelengths and to tune to receive only a selected wavelengthsignal in response to a control signal 262. In this example, tunablefilters 248 selectively tune to a particular wavelength or range ofwavelengths based on control signal 262 from control network 260.Control network 260 may comprise, for example, an Ethernet or othersuitable network or combination of communication links operable tocommunicate an electronic control signal 262. Alternatively, controlnetwork 260 could comprise control lasers operable to communicateoptical control signals 262 via switching fabric 240.

In this embodiment, optical transmitters 246 reside on their associatedline cards 230, while tunable filters reside remotely from line cards230. In this example, tunable filters 248 and switching fabric 240comprise a router core 245 for router 256. In this embodiment, routercore 245 includes switching fabric 240 combined with closely coupledtunable filters 248. Removing tunable filters 248 from line cards 236and integrating those filters into router core 245 can providesignificant advantages. For example, removing tunable filters 248 fromline cards 236 provides additional space on each line card for otherprocessing elements, or facilitates reducing the physical size of eachline card. This allows for additional line cards to reside in any givenrack. Moreover, integrating filters 248 within router core 245 at ornear switching fabric 240 facilitates the use of arrays of filters,rather than individually packaged filters for each channel. Couplingswitching fabric 240 to an array of tunable filters can significantlyreduce packaging costs and, thus, the overall cost of the router.

Filters 248, in this example, are coupled to switching fabric 240 usingoptical connections 255. Each optical connection 255 may comprise, forexample, a short length of fiber or a planar waveguide. In theillustrated embodiment, each of communication links 243 coupling opticaltransmitters 246 to switching fabric 240 comprises a single mode fiber.Communication links 245 coupling filters 248 to line cards 230 maycomprise single mode or multi-mode fibers. Communication networks usingstar couplers have traditionally used single mode fibers to couplenetwork elements both to and from the star coupler. One aspect of theinvention recognizes that in certain embodiments, such as where filters248 reside remotely from line cards 230, the use of multi-mode fibers tocouple one or more filters 248 to associated line cards 230 can providean advantage of reducing cost of router 212 without significantlydegrading performance of the device.

As discussed above, star switching fabric 40 can assume any of a varietyof physical embodiments. For example, a plurality of fibers can bephysically fused together to provide star switching capabilities. Inaddition, wave guide star couplers and arrayed wave guide gratings canbe used to provide star switching functionality. FIGS. 4A–4B depict twoparticular embodiments of novel star switching architectures that can beimplemented in any system using star switching functionality, includingthe optical routers described herein. In particular, FIG. 4A shows awavelength-based star switching fabric 40 a. Wavelength-based starswitching fabric 40 a includes a wavelength division multiplexer 41.Wavelength division multiplexer 41 receives a plurality of individualwavelength signals and combines those signals into a wavelength divisionmultiplexed signal. Wavelength division multiplexer 41 may receiveindividual wavelength signals, for example, from line cards at inputports to a router, or may receive express lane traffic directly frominput ports to the router.

Wavelength-based switching fabric 40 a includes at least one opticalamplifier 43 operable to receive and amplify the wavelength divisionmultiplex signal generated by wavelength division multiplexer 41.Optical amplifier 43 could comprise any of a variety of amplifier types,such as a distributed Raman amplifier, a discrete Raman amplifier, arare earth-doped amplifier, a semiconductor amplifier, or a combinationof these or other types of amplifiers. Amplifier 43 can be selected, forexample, to offset losses associated with distributing signals throughstar switching fabric 40 and/or to provide unity gain for bypass traffictraversing router 12.

Wavelength-based switching fabric 40 a also includes a cascade ofsplitters 45. Cascade of splitters 45 is operable to receive thewavelength division multiplexed signal from amplifier 43 and to splitthat signal into a plurality of output signals. In a particularembodiment, each splitter in cascade 47 operates to approximatelyequally split each signal received into two output signals, eachcomprising substantially the same wavelength set output from wavelengthdivision multiplexer 41. Multiple wavelength signals are thencommunicated from the outputs of cascade 47 to output links of therouter or back to line cards for further processing.

In operation, wavelength-based star switching fabric 40 a receives aplurality of signals each having a distinct center wavelength. Some ofthese signals can be the result of signals generated at line cardswithin a router, while others may be express traffic designated to passthrough the router without electrical processing. Wavelength divisionmultiplexer 41 combines some or all of these wavelengths into a multiplewavelength signal. The multiple wavelength output signal is amplified byamplifier 43 and communicated to a cascade 47 of splitters 45. Cascade47 separates the incoming multiple wavelength signal into a plurality ofoutput signals each carrying a substantially similar set of wavelengthsas the input signal to the cascade.

FIG. 4B shows another embodiment of a star switching architecture, inthis case a power-based star switching fabric 40 b. Power-based starswitching fabric 40 b includes a power combiner 44 operable to receive aplurality of input signals. In this particular example, some or all ofthe input signals have center wavelengths distinct from other inputsignals. Power combiner 44 combines the input signals based on theirpower to create a combined signal carrying all information received atthe inputs of power combiner 44. Power-based star switching fabric 40 balso includes at least one optical amplifier 46 operable to receive thecombined signal from power combiner 44, to amplify that signal, and tocommunicate the amplified signal to a power splitter 48. Amplifier 46may be similar in structure and function to amplifier 43 described withrespect to FIG. 4A. Power splitter 48 comprises a device, or combinationof devices operable to separate the power combined signal into aplurality of output signals each containing substantially the same setof wavelengths output by power combiner 44. Signals output by powercombiner 48 may be communicated directly to output links of a router, ormay be communicated to line cards for additional processing.

To resolve contention between signals competing for the same systemresources, it is helpful to implement a scheduling mechanism for usewith star switching fabrics. Although complex scheduling mechanisms canbe implemented without departing from the scope of the invention, thefollowing figures address relatively simple scheduling mechanisms thatcan be implemented in conjunction with any star switching fabric,including those described herein. These scheduling mechanisms provideadequate contention resolution capabilities while utilizing minimumprocessing resources.

FIG. 5 a is a block diagram showing one example of a schedulingmechanism 300 useful in conjunction with any star switching fabric. Thisexample depicts scheduling mechanism 300 operating within router 112shown in FIG. 2. Scheduling mechanism 300 could, however, be useful withany router or switch using a star switching fabric. In this particularexample, scheduling mechanism 300 includes a scheduling star switchingfabric 340 configured to receive input signals 252. Signals received atinputs to scheduling star switching fabric 340 comprise a non-uniformload distribution, where some inputs receive more traffic than others.In a particular example, each input to scheduling star switching fabric340 is associated with a particular wavelength and operates to receivetraffic corresponding to the associated wavelength. In one particularexample, each of the inputs to scheduling star switching fabric 340 mayreceive input optical router signals from an associated line card 230.

Scheduling star switching fabric 340 communicates signals 235 to atransmission star switching fabric 240. Transmission star switchingfabric 240 communicates output router signals 254 toward line cards 230and/or output links 228 from router 112. Scheduling star switchingfabric 340 facilitates creating a more uniform load distribution ofwavelength signals at the input to transmission star switching fabric240 compared to the load distribution received at scheduling starswitching fabric 340. Scheduling star switching fabric 340 helps to moreevenly distribute the traffic load across the inputs to transmissionswitching fabric 340 to allow scheduling of communication throughswitching fabric 240 using a relatively trivial scheduling algorithm.

Scheduling mechanism 300 includes one or more scheduling engines 364.Scheduling engine 364 comprises any hardware, software, firmware, orcombination thereof operable to instruct operation of tunable switchingelements, such as tunable transmitters or tunable filters, within router112. In this particular example, scheduling engine 364 communicatescontrol signals to a plurality of tunable filters 348 in scheduling starswitching fabric 340 and to a plurality of tunable filters 248 intransmission star switching fabric 240. Although this exampleillustrates a single scheduling engine communicating with filters 248and filters 348, separate scheduling engines could be implemented.

Scheduling engine 364 executes a scheduling algorithm to determine theorder in which filters 248 and 348 will be operated and the centerwavelength to which each filter will tune. In this particular example,scheduling engine 364 executes a trivial control algorithm, such as around robin algorithm. A round robin scheduling algorithm is simple toimplement and requires minimal system resources for execution. Roundrobin scheduling algorithms exhibit good throughput for approximatelyuniform traffic patterns. A single stage round robin scheduling schemeused in combination with a star switching fabric can, however,experience a 1/N delay when confronted with N channels of non-uniformtraffic.

One embodiment overcomes this difficulty by using one or more initialscheduling stages of scheduling star switching fabric to establish moreuniform traffic at the inputs to a transmission star switching fabric240. In particular, scheduling engine 364 instructs each of filters 348to tune to alternating wavelengths so that no one of the outputs fromscheduling star switching fabric 340 overwhelms transmission starswitching fabric 240 with any particular wavelength signal. For example,on a first pass, each of filters 348 a–348 n may communicate in roundrobin fashion optical router signals 245 having wavelengths λ₁–λ_(n)respectively. On a second pass, each of filters 348 a–348 n−1 maycommunicate in a round robin fashion optical router signals 245 havingwavelengths λ₂–λ_(n), respectively, while filter 348 n communicatessignal 245 having wavelength λ₁. Filters 348 a–348 n can continue tocycle through wavelengths λ₁–λ_(n) so that the wavelength signals 245are more uniformly distributed to the input of transmission starswitching fabric 240. Although the illustrated embodiment depicts asingle stage of scheduling star switching fabric, multiple schedulingstar switching fabrics could be cascaded to further normalize the loaddistribution entering transmission switching fabric 240.

Establishing a more uniform traffic pattern at the input of transmissionstar switching fabric 240 allows the use of a round robin algorithm tocontrol filters 248 associated with transmission star switching fabric240 without the 1/N delay penalty. Thus, scheduling mechanism 300provides a way to schedule non-uniform traffic, such as packet traffic,using a trivial scheduling algorithm for the transmission fabric, whichoccupies minimal system resources while avoiding 1/N delay penaltiestraditionally associated with simple routing algorithms and non-uniformtraffic.

Numerous modifications can be made to the example discussed with respectto FIG. 5 a. For instance, this example shows tunable filters 248 and348 as residing in close proximity to or integrally to their respectiveswitching fabrics 240 and 340. This provides an advantage of savingspace, for example, on line cards in router 112. Moreover, thistechnique provides an advantage of facilitating the economical use ofarrays of filters rather than individually packaged filters for eachoutput link. Filters 248 and 348 could, however, reside remotely fromswitch fabrics 240 and 340.

In addition, although this example shows the use of tunable filters 248and 348, tunable optical transmitters could alternatively be used inconjunction with fixed wavelength or tunable wavelength filters 248and/or 348. FIG. 5 b is a block diagram illustrating an exampleembodiment of a scheduling mechanism 305 implementing tunable opticaltransmitters as selecting elements for the scheduling star switchingfabric.

Scheduling mechanism 305 includes a plurality of tunable opticaltransmitters 346 a–346 n, which feed into scheduling star switchingfabric 340. Each tunable optical transmitter could reside, for example,on a line card within router 112. Scheduling mechanism 305 also includesa plurality of filters 348 a–348 n. In this particular example, filters348 comprise fixed wavelength filters, each associated with a particularcenter wavelength. Filters 348, in this example, reside withinscheduling star switching fabric 340. Filters 348, however, could resideremotely from switching fabric 340.

In this embodiment, outputs of filters 348 are coupled to inputs of atransmission star switching fabric 240. Transmission star switchingfabric 240 is associated, in this example, with a plurality of tunablefilters 248 a–248 n, each associated with an output link 254 a–254 nfrom the router.

Scheduling mechanism 305 further includes one or more scheduling engines364. Scheduling engine 364 instructs selecting elements 346 and 248 asto the order of tuning and the center wavelength appropriate for tuning.Although a single scheduling engine 364 is depicted, separate enginescould be implemented for elements 346 and 248.

In operation, tunable optical transmitters 346 a–346 n generate opticalsignals 252 a–252 n having center wavelengths determined by schedulingengine 364. Scheduling star switching fabric receives signals 252 a–252n and communicates substantially similar sets of at least some of thosesignals to each of filters 348. In this example, each filter comprises afixed wavelength filter operable to pass signals having a particularcenter wavelength.

Signals 235 passed by filters 348 are then communicated to transmissionstar switching fabric 240. tunable filters 248 of transmission starswitching fabric 240 tune to receive selected wavelengths according toinstructions from scheduler 364. As a result, selected wavelengthsignals are passed from transmission star switching fabric 240 to outputlinks 254.

FIG. 5 c is a block diagram illustrating another example of a schedulingmechanism 310 useful in conjunction with any star switching fabric. Likethe example shown in FIG. 5 a, this example depicts scheduling mechanism310 operating within router 112 shown in FIG. 2. Scheduling mechanism310 could, however, be useful with any router or switch using a starswitching fabric. Scheduling mechanism 310 is similar in structure andfunction to scheduling mechanism 310 shown in FIG. 5 a.

Scheduling mechanism 310 implements a buffering stage 230 betweenscheduling star switching fabric 340 and transmission star switchingfabric 240. Buffering stage 230 facilitates synchronization and aids inscheduling communications between scheduling star switching fabric 340and transmission star switching fabric 240. As a particular example,buffering stage 230 could comprise a plurality of line cards, eachassociated with an input to transmission star switching fabric 240.Buffering stage 230 may also include memory used to avoid missequencingof packets received by and communicated from scheduling star switchingfabric 340.

In this example, scheduling switching fabric 340 receives the multiplewavelength signal from input link 222 and communicates separatewavelength signals 228 a–228 n (along with any express traffic 228 ex)from switching fabric 340. In the illustrated embodiment, wavelengthsignals 228 a–228 n are communicated to line cards 230 for bufferingand/or electronic decision making with respect to routing those signalsthrough switching fabric 240. Transmission star switching fabric 240receives input router signals 252 a–252 n and communicates those signalstoward destination elements associated with those signals.

Scheduling switching fabric 340 operates to separate the multiplewavelength signal received at input 222 into a plurality of wavelengthsignals each having a center wavelength. In this particular example,Scheduling switching fabric 340 includes or is closely coupled to aplurality of tunable filters 348 a–348 n, and 348 ex. Tunable filters348 selectively pass wavelength signals 228 a–228 n toward transmissionstar switching fabric 240. In this particular embodiment, schedulingstar switching fabric 340 passes selected signals 228 to line cards 230for processing.

Like the example in FIG. 5 a, scheduling engine 364 operates toprovision tunable filters 348 a–348 n in a round-robin fashion so thateach filter 348 alternates the wavelength it passes toward transmissionstar switching fabric 240. In this manner, scheduling switching fabric340 operates to make non-uniform traffic received at input 222 moreuniform at the inputs to transmission star switching fabric 240. Becausethe incoming signals 252 a–252 n to switching fabric 240 are moreuniform in load distribution, scheduling mechanism 310 a can ensurereasonable throughput through switching fabric 240 while utilizing arelatively simple scheduling algorithm, such a round-robin schedulingalgorithm.

The particular embodiment shown in FIG. 5 c is just one example of animplementation of scheduling mechanism 310 in an optical router. Variousmodifications can be made without departing from the scope of thisaspect of the invention. For example, rather than using tunable filtersin both switching fabrics 240 and 340, tunable lasers could beimplemented in conjunction with fixed or tunable filters to achievesimilar operational effects. For example, line cards 230 could includetunable lasers operable to selectively communicate optical routersignals 252 a–252 n at selected wavelengths to fixed wavelengthtransmitters 248 a–248 n associated with particular output links fromrouter 112.

Moreover, although this example shows filters 248 and 348 as residingintegrally to or in close proximity with switching fabrics 240 and 340,respectively, filters 248 and/or 348 could alternatively reside remotelyfrom their associated switching fabrics. In one particular example,filters 248 and/or 348 could reside on line cards associated with thosefilters, or in another location remote from their associated switchingfabrics.

As another example of a potential modification to the embodiment shownin FIG. 5 c, processing capabilities and look-up tables of line cards230 could be eliminated, while electronic or optical memory structuresresident on the line cards could remain. These memory structures couldserve as buffers to optical signals received from scheduling switchingfabric 340 and awaiting transmission to transmission switching fabric240. These buffers could further enhance the uniformity of wavelengthscommunicated to star switching fabric 240.

FIG. 5 d is a block diagram showing yet another example of a schedulingmechanism 320 useful in conjunction with any star switching fabric. Likethe example shown in FIGS. 5 a–5 c, this example depicts a schedulingmechanism 320 operating within router 112 shown in FIG. 2. Schedulingmechanism 320 could, however, be useful with any router or switch usinga star switching fabric.

Scheduling mechanism 320 is similar in structure and function toscheduling mechanism 320 shown in FIG. 5 c. Scheduling mechanism 320,however, implements an input buffer stage 330 operable to receivewavelength signals from a wavelength division multiplexer 325 and anoutput buffer stage 332 operable to operable to receive wavelengthsignals 254 output from transmission star switching fabric 240.

Input buffer stage 330 facilitates segmentation, synchronization,buffering, and/or scheduling of communications to scheduling starswitching fabric 340. Input buffer stage 330 could comprise anyhardware, software, firmware, or combination thereof operable tofacilitate storage and retrieval of signals received. In someembodiments, input buffer stage 330 could comprise an optical memorycomprising, for example, one or more delay loops. In other embodiments,input buffer stage could comprise an electronic memory. Input bufferstage 325 could reside, for example on one or more line cards operableto convert at least a portion of incoming optical signals to anelectronic format and to generate optical signals for retransmission toscheduling switching fabric 340. In one particular embodiment, inputbuffer stage 325 could reside on line cards 230.

Input buffer stage 325 can facilitate creating an even more uniform loaddistribution of wavelength signals at the input to star switching fabric240. Moreover, input buffer stage 325 can provide a mechanism to helpalleviate missequencing of packets at the outputs from star switchingfabric 240. This technique can be particularly effective when used incombination with a Full Frames First algorithm to control the buffers inthe system.

In operation, scheduling mechanism 320 receives at wavelength divisionmultiplexer 325 a multiple wavelength input signal from input link 222.Wavelength division multiplexer 325 separates the multiple wavelengthinput signal into a plurality of optical signals, each having a centerwavelength. Input buffer stage 325 stores incoming wavelength signalsuntil those signals are communicated toward scheduling switching fabric340. Switching fabric 340 communicates substantially similar sets ofsome or all of the wavelength signals received to filters 348.

In this example, filters 348 comprise tunable filters residing in closeproximity to or integrally with switching fabric 340. Scheduling engine364 instructs each of filters 348 a–348 n in a round robin fashion toalternately communicate signals having various selected wavelengths.This reduces the nonuniformity of wavelengths of incoming signals.

Transmission star switching fabric 240 receives input router signals 252having a more uniform load distribution, and communicates substantiallysimilar sets of some or all of the wavelength signals received tofilters 248. Each of filters 248 is provisioned in a round robin fashionto pass selected wavelength signals toward output links associated withappropriate destination elements.

As in the examples described in FIGS. 5 a–5 c, the example shown in FIG.5 d could be modified in any number of ways. For example, tunableoptical transmitters could be used in place of some or all of thetunable filters implemented. Moreover, filters 248 and 348 could resideremotely from their associated switching fabrics.

Each of the embodiments of scheduling mechanisms depicted in FIGS. 5 a–5d provides a way to provide adequate throughput through switching fabric240 while utilizing a relatively simple scheduling algorithm.

FIGS. 6 a–6 d provide additional nonlimiting examples of implementationsof scheduling mechanisms useful with star switching fabrics. FIG. 6 a isa block diagram showing an example of a multiple buffer embodiment 315utilizing tunable optical filters as selecting elements within ascheduling star switching fabric. In particular, embodiment 315 includesa plurality of line cards 230 which serve as an input buffer stage 230a, an intermediate buffer stage 230 b, and an output buffer stage 230 c.Although this embodiment depicts the use of different sets of cards 230a–230 c to serve as input, intermediate, and output buffer stages, thesame set of line cards could likewise be used for some or all of thebuffer stages, or one or more buffer stages could be eliminated.

In this example, input buffer stage 230 a operates to segment incominginformation into, for example, fixed length frames or cells fortransmission through transmission switching fabric 240. Input bufferstage 230 a can also perform a temporary storage function while packetsare scheduled for transmission through scheduling star switching fabric340.

In the illustrated embodiment, scheduling star switching fabric 340comprises or is coupled to a plurality of tunable optical filters 348a–348 n, each associated with an output from scheduling star switchingfabric 340. Under the control of a scheduling engine 364 (located, forexample, on one or more line cards 230), tunable filters 348 a–348 ntune, in a round robin fashion, to particular wavelengths to betransmitted toward the inputs of transmission star switching fabric 240.Scheduling engine 364 instructs each filter 348 to alternate thewavelength of information communicated so that the inputs totransmission star switching fabric 240 experience a more uniform trafficload than the inputs to scheduling star switching fabric 340.

In this example, optical transmitters associated with each line card 230b generate input optical router signals 252 at particular wavelengthsassociated with each line card 230 b. Signals 252 are communicated totransmission star switching fabric 240, where substantially similar setsof at least some of input optical router signals 252 are communicated toeach of a plurality of tunable filters 248 a–248 m, each associated withan output link from the device. Processors on or associated with linecards 230 b perform electronic decision making on signals 228 receivedto determine an appropriate path for each signal from transmission starswitching fabric 240. Based on this determination, the processorsinstruct tunable filters 248 to tune to particular wavelengths so thatsignals destined for the output link associated with that tunable filter248 are passed by that filter.

Because scheduling star switching fabric 340 has created a more uniformtraffic distribution at the inputs of transmission star switching fabric240, the scheduling engine that schedules communication throughtransmission star switching fabric 240 can implement a trivialscheduling algorithm, such as a round robin algorithm, to effectivelyadminister system resources.

FIG. 6 b is a block diagram of an example multiple buffer embodiment 316utilizing tunable optical transmitters 346 as selecting elements withina scheduling star switching fabric 340.

Embodiment 316 includes a plurality of line cards 230 which serve as aninput buffer stage 230 a, an intermediate buffer stage 230 b, and anoutput buffer stage 230 c. Buffer stages 230 a–230 c can serve similarfunctions to like stages described above with respect to FIG. 6 a.Although this embodiment depicts the use of different sets of cards 230a–230 c to serve as input, intermediate, and output buffer stages, thesame set of line cards could likewise be used for some or all of thebuffer stages, or one or more buffer stages could be eliminated.

In the illustrated embodiment, scheduling star switching fabric 340comprises or is coupled to a plurality of tunable optical transmitters346 a–346 n, each associated with an input to scheduling star switchingfabric 340. Under the control of a scheduling engine 364 (located, forexample, on one or more line cards 230), tunable transmitters 346 a–346n tune, in a round robin fashion, to particular wavelengths to betransmitted toward the inputs of scheduling star switching fabric 340.Scheduling engine 364 instructs each transmitter 346 to alternate thewavelength of information communicated so that the inputs totransmission star switching fabric 240 (received from outputs ofscheduling star switching fabric 340) experience a more uniform trafficload than the inputs to scheduling star switching fabric 340.

Scheduling star switching fabric 340 receives the plurality of incomingsignals and communicates substantially similar sets of at least some ofthe signals received to each of a plurality of fixed wavelength filters348 a–348 n. Each filter 348 is tuned to a particular wavelength andcommunicates signals 228 having the associated wavelength to anassociated one of line cards 230.

Processors on or associated with line cards 230 b perform electronicdecision making on signals 228 received to determine an appropriate pathfor each signal from transmission star switching fabric 240. In thisexample, each line card 230 includes or is associated with a tunableoptical transmitter 246 a–246 n, respectively. Tunable opticaltransmitters 246 tune to selected wavelengths under the direction ofscheduling engine 364 executed by the processors. Scheduling engine 364instructs each tunable transmitter 246 to tune, in a round robinfashion, to a particular wavelength. Signals are communicated fromtunable transmitters 246 to transmission star switching fabric 240,which communicates a substantially similar set of at least some of thesignals received toward each of a plurality of fixed wavelength filters248 within or coupled to transmission star switching fabric 240. Thewavelength selected for each transmitter will determine the output linkover which the generated signal will pass, as each of the fixedwavelength filters 248 passes a wavelength associated with a particularoutput link associated with that filter.

Because scheduling star switching fabric 340 has created a more uniformtraffic distribution at the inputs of transmission star switching fabric240, the scheduling engine that schedules communication throughtransmission star switching fabric 240 can implement a trivialscheduling algorithm, such as a round robin algorithm, to effectivelyadminister system resources.

FIG. 6 c is a block diagram showing yet another embodiment 317 of amultiple buffer stage switching fabric using tunable optical filters asselecting elements.

This embodiment is similar in structure and function to the embodimentdepicted in FIG. 6 a, but introduces input signals directly toscheduling star switching fabric 340 without an input buffer stagepreceding scheduling star switching fabric 340.

FIG. 6 d is a block diagram showing yet another embodiment 318 of amultiple buffer stage switching fabric using tunable optical filters asselecting elements.

This embodiment is similar in structure and function to the embodimentdepicted in FIG. 6 c, but implements a power combiner 333 in place ofwavelength division multiplexers 210 shown in FIGS. 6 a–6 c. Inaddition, this embodiment can use one or more optical amplifiers 337prior to the input to scheduling star coupler 340. Optical amplifiers337 operate to compensate for at least a portion of the loss otherwisecaused by power combiner 333.

As discussed above, various embodiments of devices implementing starswitching fabrics implement optical transmitters to generate signalsdestined for the star switching fabric. Some embodiments describedherein have discussed implementing optical transmitters having fixed ortunable wavelength capabilities on line cards within the devices. As thenumber of channels serviced by the system increases, difficulties canarise with respect to implementation of conventional optical transmittertechnology.

For example, implementing a conventional laser diode on each line cardservicing a transmission channel can be prohibitively expensive as thenumber of channels become large. Moreover, conventional lasers andassociated control circuitry can take up significant space on each linecard, leaving less space for other processing elements, or requiringlarger line cards. Requiring larger line cards typically reduces thenumber of cards that can be placed in any given rack.

In addition, as the number of channels increases, it becomesincreasingly difficult to administrate accurate assembly of line cardsusing fixed wavelength transmitters. In that case, it becomes necessaryto ensure that each line card receives a transmitter operating at awavelength specified for that card. Increased numbers of channels makeit difficult to accurately associate transmitter part numbers withparticular line cards.

Furthermore, as the number of channels increases, the channel spacingtypically becomes more narrow. It becomes increasingly difficult tostabilize the wavelength of each individual transmitter to ensure properchannel spacing.

FIG. 7 is a block diagram of an optical transmitter system 380particularly useful, for example, in conjunction with a star switchingfabric implementing large number of channels, for instance 64 or morechannels. Optical transmitter system 380 comprises a continuum source.In a particular embodiment, system 380 could comprise a supercontinuumsource. Supercontinuum generation describes extreme, nearly continuousspectral broadening induced by high-intensity picosecond andsub-picosecond pulse propagation through a nonlinear medium.

In this example, system 380 includes a modelocked source 382 operable togenerate a series of optical pulses. As a particular example, modelockedsource 382 could comprise an erbium doped fiber laser operable togenerate pulses at a rate of, for example, forty gigabits per second.Other modelocked sources operating at other rates could likewise beused.

System 380 further includes a continuum generator 384 operable toreceive a train of pulses from modelocked source 382 and to spectrallybroaden the pulses to form an approximate spectral continuum of opticalsignals. In this example, continuum generator 384 includes an opticalamplifier 383 coupled to one or more lengths of optical fiber 385.Optical amplifier 383, in this particular example, comprises an erbiumdoped amplifier. Other amplifier types or combinations of amplifiertypes could likewise be used. In this example, fiber 385 comprises a twostage solition-effect compressor including approximately two meters ofstandard fiber followed by approximately two meters of dispersionshifted fiber. Other lengths of fiber and fiber types could be used,depending on the spectral characteristics desired. Moreover, althoughthis example relies on the solition effect to broaden the spectrum ofthe plurality of optical pulses, other pulse compression techniques,such as adiabatic solition compression, could alternatively be used.

System 380 also includes a signal splitter 386. Signal splitter 386receives the continuum from continuum generator 384 and separates thecontinuum into individual signals 389 a–389 n each having a wavelengthor a range of wavelengths. Signal splitter 386 could comprise, forexample, a passive wavelength division multiplexer, a power splitterfollowed by fixed wavelength filters, or any other mechanism operable toseparate a continuum or near continuum of signals into a plurality ofindividual signals.

Mode locked source 382, continuum generator 384, and signal splitter 386can comprise common bay equipment—in other words, equipment shared byplurality of line cards 390. Where it is desired to generate a largerbandwidth of optical signals, multiple sets of common bay equipment 381can be implemented, each set serving a separate set of line cards 390and each generating a separate range of wavelengths.

Signal splitter 386 communicates signals 389 a–389 n to one of aplurality of modulators 392 a–392 n, respectively. Modulators 392operate to encode information onto the optical signals received toproduce optical wavelength signals 393 for transmission to a starswitching fabric. In this particular example, each modulator 392 resideson a line card 390. When used with a continuum source, each of theplurality of transmitters in system 380 can be viewed as one ofmodulators 392 in combination with equipment, such as common bayequipment 381, used to generate the unmodulated signal received by eachmodulator 392.

In some embodiments, system 380 further comprises a pulse ratemultiplexer 387, such as a time division multiplexer. Pulse ratemultiplexer 387 operates to multiplex pulses received from mode lockedsource 382 to increase the bit rate of the system. Pulse ratemultiplexer 387 could alternatively reside downstream from modulators392 and operate to time division multiplex signals received frommodulators 392.

In operation, modelocked source 382 generates a plurality of opticalpulses at a given rate. Continuum generator 384 receives the train ofpulses from modelocked source 382 and compresses those pulses to form anapproximate continuum of optical signals. Signal splitter 386 receivesand separates the continuum into a plurality of optical signals 389a–389 n each comprising a wavelength or range of wavelengths. Eachmodulator 392 receives one of signals 392 from signal separator 386 andencodes information onto the optical signal received to generate signals393 for transmission to a star switching fabric.

Transmitter system 380 can support generation of fixed wavelengthsignals or selectively tuned wavelength signals. To facilitategeneration of selectively tuned wavelength signals, system 380 couldinclude, for example, a signal selector 395 operable to selectively passparticular wavelength signals to particular modulators 393, depending onthe wavelength signal desired to be transmitted from that modulator 393.Signal selector 395 could comprise any hardware, software, firmware, orcombination thereof operable to send particular wavelength signals toparticular modulators in response to, for example, a control signalgenerated by a scheduling engine.

System 380 provides numerous benefits over systems implementing separateoptical transmitters for each channel. For example, implementing one ormore common modelocked sources to generate numerous wavelength signals,saves considerable space on each line card, and reduces cost byeliminating numerous individual transmitters. Moreover, system 380facilitates using common parts, such as modulators, for a number ofdifferent line cards serving different channels. This makes it easier tomatch parts to each line card. Furthermore, stabilization issues can bealleviated because system 380 allows stabilization of one or a fewcommon transmitter elements, rather than requiring stabilization ofseparate transmitters each associated with one of the channels.

FIGS. 8–9 are block diagrams illustrating example mechanisms usefulenhancing the effective switching speed of devices using star switchingfabrics. For the purposes of illustration, these mechanisms will bedescribed with reference to router 112 shown in FIG. 2. These mechanismscould, however, equally apply to many other device designs implementingstar switching fabrics.

FIG. 8 a illustrates the use of a speed-up mechanism 125 at line card130. In this example, line card 130 receives incoming optical signal128, which includes packets having a first duration, say fiftynanoseconds each. Each optical packet 128 is converted to an electronicsignal within line card 130 and then placed into an optical format 152for transmission to the router switching fabric.

Speed-up mechanism 125 of line card 130 operates to decrease theduration of each optical packet 128. For example, speed-up mechanism 150may increase the speed at which a modulator of line card 130 encodesinformation onto optical signal 152. As a particular example,information can be modulated onto optical signal 152 at an increasedrate resulting in the information received in optical signal 128 beingmodulated in an optical signal 152 having one half the duration ofsignal 128. Other speed-up ratios could be used without departing fromthe scope of the invention.

FIG. 8 b is a block diagram showing one example of an aggregator 135operable to aggregate a plurality of incoming packets 131 into a singleaggregated frame 137. Each aggregated frame includes an identifieridentifying a destination element common to each packet 131 in theaggregated frame 137. Aggregator 135 can aggregate multiple packets 131,for example, by encapsulating a plurality of packets within a singleaggregation frame having a common aggregation header.

Aggregator 135 can assemble aggregated frames 137 in a variety of ways.For example, aggregator 135 can aggregate optical packets received atline card 130 from input link 128, associating an identifier with eachframe 137. Line card 130 can then convert at least the identifierportion of the frame 137 to an electronic format to facilitateelectronic processing of that information. Line card 130 could thengenerate an optical aggregation header and reform an aggregated framefor transmission to star switching fabric 140. As another example,aggregator 135 could form aggregated frames 137 after each packet 131 ofthat frame or portions thereof are processed by processor 136. In thatcase, processor 136 converts all or a portion of each packet received toan electrical signal to facilitate electronic processing. Transmitter146 forms optical router packets, and aggregator 135 combines opticalrouter packets into aggregated frames 137.

Allowing switching fabric 140 to switch a smaller number of largerframes rather than numerous individual packets can provide significantswitching efficiencies.

FIGS. 9 a–9 c are block diagrams showing various embodiments of filterand transmitter configurations operable to enhance the effectiveswitching speed of router 12 without modifying the switching speed ofany individual components, such as filters 148 or transmitters 146. Inparticular, FIG. 9 a is a block diagram of a multiple filterconfiguration. The speed of router 112 can be limited in some cases bythe switching speed of filters 148. That is, each filter requires somefinite time to tune between different wavelengths desired to beprocessed. If router 112 is forced to wait while filters 148 resetbetween wavelengths, the speed of router 112 can be significantlyhindered.

The example embodiment in FIG. 9 a helps to alleviate this problemwithout requiring increased switching speed of any one filter 148, byassigning a plurality of filters 148 a 1–148 ax to each optical link128. Filters 148 a 1–148 ax operate in parallel so that while one filter148 a 1 is processing output optical router signal 154 from switchfabric 140, other filters 148 a 2–148 ax can be retuned to anotherwavelength to receive packets carried over other channels. By switchingbetween the multiple parallel filters 148 a 1–148 ax, switching delaythat might otherwise be caused when waiting for filters 148 to retunecan be significantly reduced.

In the illustrated embodiment, an optical splitter 141 receives outputoptical router signal 154 from switch fabric 140 and communicates aportion 154 a 1–154 ax to each of filters 148 a 1–148 ax, respectively.In this particular example, a switch 151 cycles between signals receivedfrom filters 148 a 1–148 ax so that only one of the signals from filters148 a is output to line card 136. Although this example shows use of asequential control algorithm that switches from one filter output toanother, a variety of control algorithms can be used to determine anactive filter 148 a. For example, switch 151 could receive a controlsignal instructing switch 151 as to which filter output to accept.

In the embodiment shown in FIG. 9 a, optical signals output from filters148 a are converted to electrical signals at receivers 149 a, eachassociated with one of filters 148 a. Switch 151 operates to processelectrical signals received from converters 149 a and to pass anelectrical output to an associated line card 136. The embodiment shownin FIG. 9 b is similar in structure and function to that shown in FIG. 9a, except that electrical switch 151 is replaced with an optical switch153. Optical switch operates to receive optical signals from filters 148a and to select one of those optical signals for communication toconverter 149 a. Converter 149 a converts the selected signal to anelectrical signal and passes the converted electrical signal to anassociated line card 136.

Although the example shown in FIG. 9 b depicts the use of multiplefilters per line card 136, a similar concept could be applied to filtersassociated with express channels 127. In that case, converters 149 couldbe eliminated so that optical signals output from optical switch 153pass to wavelength division multiplexer/demultiplexer 110 from opticallink 127.

FIG. 9 c is a block diagram showing yet another mechanism operable toreduce switching delay of router 112 without modifying the switchingspeed of individual switching components. This example implements aplurality of tunable lasers 146 a 1–146 ax associated with each linecard 136.

While one of optical transmitters 146 a 1–146 ax generates an opticalrouter signal having one particular wavelength, other transmitters 146 a2–146 ax can be retuned to another wavelength to communicate packetsbound for other destinations. By switching between the multiple paralleltransmitters 146 a 1–146 ax, switching delay that might otherwise becaused when waiting for transmitters 146 to retune can be reduced oravoided.

In the illustrated embodiment, a splitter 143 receives electrical signal129 a from processor 136 and communicates a portion 129 a 1–129 ax toeach of transmitters 146 a 1–146 ax, respectively. At least one oftransmitters 146 a 1 generates an optical router signal at a specifiedwavelength. Other transmitters 146 a 2–146 ax can retune withoutemitting light during the time that active transmitter 146 a 1 generatesthe optical signal. For example, where optical transmitters 146 comprisemultiple stage lasers including tuning stages and lasing stages, thelasing stages of those transmitters can remain inactive while tuningstages adjust to process a new wavelength.

A switch 155 selects an appropriate optical router signal from lasers146 a and communicates that signal to switching fabric 140. In oneembodiment, switch 155 can sequentially cycle between signals receivedfrom transmitters 146 a 1–146 ax. A variety of control algorithms can beused to determine an active transmitter 146 a. For example, switch 155could receive a control signal instructing switch 155 as to whichtransmitter output to accept and communicate to switch fabric 140.

Each of the efficiency enhancing mechanisms described with respect toFIGS. 4 and 5 could be used independently or in combination with one,some, or all others of those mechanisms to further enhance operation ofthe router.

FIG. 10 is a flow chart illustrating one example of a method 400 ofrouting optical signals. For the purposes of illustration, method 400will be described with reference to router 112 shown in FIG. 2. Method400, however, could equally apply to alternative router designs, such asrouter 212 shown in FIG. 3 including enhancements shown in FIGS. 8and/or 9. Method 400 begins at step 405 where line card 130 a receives afirst packet 128 a comprising an identifier of a destination element.

Processor 136 a of line card 130 a converts at least the identifierportion of first packet 128 a to an electronic format. Processor 136 aapplies the identifier to look-up table 144 a to determine controlsignal 162 a at step 410. In this example, control signal 162 ainstructs a particular tunable filter, for example, filter 148 n to tuneto a wavelength transmitted by transmitter 146 a of first line card 130a. Alternatively, processor 136 a could communicate control signal 162to scheduling engine 164 to facilitate scheduling and arbitration amongcontrol signals 162 before transmitting those signals to filters 148.

The identification of a destination tunable filter 148 could compriseidentification of a plurality of tunable filters operating in parallelto service a single optical link and/or line card. Embodiments discussedwith respect to FIGS. 9 a–9 b provide examples of this type ofoperation. In this manner, one of the filters 148 can process theoptical signal while other filters in that group retune to or from otherwavelengths. This can help to enhance the effective switching speed ofrouter 112.

Transmitter 146 a generates an optical router signal 152 a andcommunicates that signal to star coupler switch fabric 140 at step 415.In a particular embodiment, transmitter 146 a comprises a fixedwavelength transmitter operable to generate optical router signal 152 aat a particular fixed wavelength. Generating optical router signal 152could comprise, for example, generating optical router signal 152 usinga laser/modulator combination residing on the same line card. In anotherexample, a modulator 393 resident on line card 130 a could receive fromcommon bay equipment (see e.g., FIG. 7) an unmodulated optical signalhaving a particular wavelength. Modulator 393 could modulate informationonto the unmodulated signal to generate optical router signal 152.

The process by which transmitter 146 a generates optical router signal152 a depends, in part, on the level of conversion experienced byincoming packet 128 a. Where processor 136 a converts the entire opticalsignal 128 a into an electronic format, transmitter 146 a informationfor the entire optical signal including header and payload informationfor optical router signal 152 a. Where, on the other hand, processor 136a converts only a portion of optical signal 128 a, transmitter 146 amerely converts that portion of the signal back to an optical signal,and recombines that portion with the original optical portion of signal128 a to form optical router signal 152 a. As a particular example,processor 136 a may convert only a header portion, or only theidentifier portion of a header portion of signal 128 a to an electronicformat, while temporarily storing or delaying the remainder of opticalsignal 128 a until it can be combined with an optical signal leavingtransmitter 146 a.

Generation of optical router signal 152 a may include aggregatingindividual packets 131 into larger frames 133 and/or may includereducing the duration of each packet by implementing a speed-upmechanism such as that described with respect to FIG. 8 a.

Star coupler switching fabric 140 receives the first optical routersignal 152 a and at least one other optical router signal 152 b having awavelength that is different than first optical router signal 152 a, andcommunicates both optical router signals 152 a to a plurality of tunablefilters 148 at step 420. In this example, tunable filter 148 n isassociated with a line card coupled to an optical path facilitatingcommunication with the destination network element. In this case, router112 communicates control signal 162 to tunable filter 148 n at step 425.

Based at least in part on control signal 162 a, filter 148 n associatedwith line card 130 n tunes to the wavelength associated with opticalrouter signal 152 a. As a result, filter 148 n accepts the first packetcarried by optical router signal 152 a at step 430 and facilitatescommunication of the first packet toward the destination element.Tunable filter 248 n comprises a tunable optical filter operable toselectively accept one or more specified wavelengths while rejectingothers. Filter 148 n may communicate the first packet toward thedestination element without further conversion, or may pass opticalrouter signal 152 a to an optical-to-electrical converter 149 n tofacilitate additional processing before communicating the first packettoward the destination network element.

FIG. 11 is a flow chart showing one example of a method 350 ofscheduling communications through a star switching fabric. Method 350will be described with respect to scheduling mechanism 300 shown in FIG.5 a. Method 350 could apply, however, to any scheduling mechanismdescribed herein.

Method 350 begins at step 355, where scheduler 300 receives a pluralityof packets having a first load distribution. Scheduler 300 couldreceive, for example a plurality of packets in an optical format, whereeach packet is associated with a wavelength. Typically, packet-basedtraffic will exhibit a non-uniform load distribution.

In this particular example, scheduling star switching fabric 340 ofscheduler 300 receives packets 252 a–252 n, and communicates asubstantially similar set of at least some of packets 252 toward each ofa plurality of filters 348 at step 360. In this example, filters 348each comprises a tunable filter operable to selectively tune to awavelength to be passed. Alternatively, filters 348 could comprisefixed-wavelength filters used in combination with tunable wavelengthoptical transmitters, such as transmitters 346 shown in FIG. 5 b.

Scheduler 300 selectively passes packets associated with selectedwavelengths for receipt by transmission star switching fabric 240 atstep 365. In this example, under the direction of scheduling engine 364,filters 348 selectively tune to alternating wavelengths in a round robinfashion to ensure that no one particular wavelength overwhelmstransmission switching fabric 240. The result of the selective alternatetuning of filters 348 culminates in a more uniform load at the input totransmission star switching fabric 240.

As a result, scheduler 300 schedules communication of packets fromtransmission switching fabric 240 at step 370 using a trivial schedulingalgorithm. Scheduler 300 may implement, for example, a round robinalgorithm for scheduling tuning of selectable elements, such as filters248, associated with transmission star switching fabric 240. Byestablishing a more uniform load at the input to transmission starswitching fabric 240, scheduler 300 avoids the 1/N delay penalty thatwould otherwise be associated with using a trivial scheduling algorithmon non-uniform traffic.

FIGS. 12–16 are flow charts illustrating example methods of enhancingthe effective switching speed of a router utilizing a star switchingfabric without increasing switching speed of the individual switchingcomponents of the router. For brevity of description, the followingmethods will be described with reference to router 112 depicted in FIG.2. The methods described with respect to FIGS. 12–16 could, however,apply to any router design utilizing a star switching fabric, and arenot intended to be limited only to the example router embodimentsexplicitly described herein.

FIG. 12 is a flow chart illustrating one example of a method 450 ofenhancing the effective switching speed of router by reducing theduration of packets communicated through a star switching fabric of therouter. Method 450 begins at step 455 where router 112 receives at afirst line card 130 an optical packet comprising a payload and having afirst duration. Referring to FIG. 8 a, optical packet 131 may comprise aduration of, for example, 50 nanoseconds. Line card 130 generates atstep 460, an optical router packet 133 having a second duration shorterthan the first duration. Optical router packet 133 comprises the payloadof optical packet 131 received by line card 130, and comprises a secondduration shorter than the first duration associated with packet 131. Inthis particular example, the second duration of packet 133 comprisesapproximately one half the duration of input packet 131.

Line card 130 communicates the optical router packet 133 to starswitching fabric 140 at step 465. Star switching fabric 140 communicatesat step 470 a plurality of optical router packets to each of a pluralityof tunable filters 148. Each tunable filter 148 is associated with aseparate output link from router 112. Router 112 communicates at step475 a control signal 162 to a selected tunable filter 148 to facilitatecommunicating at least the payload of the optical router packet 133toward the destination element associated with that packet. The controlsignal 162 causes tunable filter 148 to tune to a wavelength associatedwith optical packet 133, and to substantially communicate packet 133toward a destination element associated with that optical filter 148.Prior to communicating optical packet 133 from router 112, router 112may expand the duration of packet 133 to recover its original duration.

By reducing the duration of packets received at line cards 130, router112 can increase switching speed and throughput associated with therouter without modifying the switching speeds of any particularswitching components in router 112.

FIG. 13 is a flow chart showing one example of a method 500 of enhancingthe effective switching speed of an optical router by aggregatingpackets bound for a common destination element. Method 500 begins atstep 510 where router 112 receives a plurality of optical packets eachcomprising a payload and each comprising an identifier of the samedestination element. Referring to FIG. 8 b, router 112 generates at step520 an aggregated frame 137 comprising an identifier 139 of thedestination element shared by packets 131 a–131 n.

Router 112 communicates at step 530 aggregated frame 137 to starswitching fabric 140. In this example, star switching fabric 140communicates at step 540 aggregated frame 137 to each of a plurality oftunable filters 148. Each tunable filter is associated with a separateoutput link from router 112. Alternatively, aggregated frames 137 couldbe generated by tunable optical transmitters and communicated to aplurality of fixed wavelength filters through star switching fabric 140.

In the illustrated example, router 112 communicates a control signal toat least a selected tunable filter 148 at step 550. The selected tunablefilter 148 is associated with a communication path to a destinationelement for each of the optical packets 131 a–131 n within aggregatedframe 137. The selected tunable filter 148 receives a control signal andtunes to a wavelength associated with aggregated frame 137, facilitatingcommunication of aggregated frame 137 toward the destination element. Aline card 130 associated with the output link 128 leading to thedestination element may disassemble aggregated frame 137 to facilitatecommunication of individual packets 131 a–131 n toward the destinationelement.

FIG. 14 is a flow chart showing one example of a method 600 of enhancingthe effective switching speed of an optical router using a starswitching fabric by providing express lanes that bypass line cards thatfacilitate electronic signal processing of some of the optical signalsreceived. Method 600 begins at step 610 where router 112 receives aninput optical packet at optical link 128. A line card 130 converts atleast a portion of the optical packet received to an electronic form atstep 620. Line card 130 generates, based at least in part on theelectronic signal, an optical router signal having a first wavelength atstep 630.

Router 112 also receives at an express lane 127 an express opticalpacket having a second wavelength at step 640. Router 112 communicatesat step 650 the optical router packet generated at line card 130 and theexpress packet received at express lane 127 to star switching fabric140. Star switching fabric 140 communicates the optical router packetand the express packet to each of a plurality of tunable filters at step660. Router 112 communicates a control signal to a selected tunablefilter at step 670 to facilitate communicating the express opticalpacket toward a destination element associated with that filter. Theexpress optical packet is communicated from an input to router 112,through switching fabric 140, to an output of router 112 without everhaving been converted to an electronic form. Facilitating bypassing linecards 130 depending, for example, on the wavelength of the opticalpackets received, can provide significant efficiencies. Packets that donot require electronic processing can transparently pass through router112, saving system resources and reducing delay that would otherwiseaccompany having to convert all packets received between optical andelectrical formats.

Again, although this example discusses the use of tunable filters andfixed wavelength transmitters, the concepts also apply to embodimentsutilizing tunable optical transmitters and fixed wavelength filters.

FIG. 15 is a flow chart showing one example of a method 700 forenhancing the effective switching speed of an optical router using astar switching fabric by assigning a plurality of tunable filters toeach output link from the router. Method 700 begins at step 710 whererouter 112 receives at star switching fabric 114 a plurality of opticalsignals each having a wavelength. Although some of the optical signalsmay have the same wavelengths, at least some of the signals receivedhave different wavelengths from other signals received. Star switchingfabric 140 communicates at step 720 a plurality of substantially similarsets of the optical signals. In some embodiments, each of thesubstantially similar sets of optical signals may comprise a combinationof all signals received by the star switching fabric 140. In otherembodiments, star switching fabric 140 may communicate only some of theoptical signals received.

A group of tunable filters 148 associated with a common output fromrouter 112 receives one of the plurality of substantially similar setsof optical signals at step 730. Referring for example to FIG. 9 a, afirst tunable filter 148 a 1 of the group of tunable filters associatedwith the output link is tuned to a first wavelength to process one ofthe optical signals received having primarily the first wavelength atstep 740. While the first filter 148 a 1 processes the optical signalprimarily comprising the first wavelength, a second tunable filter 148an of the same group tunes to a second wavelength at step 750. In aparticular embodiment, the second tunable filter 148 an cansubstantially complete tuning to the second wavelength before the firsttunable filter 148 a 1 completes processing the optical signal havingprimarily the first wavelength.

Router 112 communicates the optical signal having primarily the firstwavelength from first tunable filter 148 a 1 to an output linkassociated with that filter at step 760. Subsequently, the group oftunable filters, and in particular, second tunable filter 148 an tunedto the second wavelength may receive another set of optical signals andfacilitate communication of an optical signal comprising primarily thesecond wavelength toward the output link associated with that group offilters.

Assigning a plurality of tunable filters to a single output link allowsrouter 112 to conceal delay that would otherwise be associated withhaving to retune filters to process different wavelength signals. Usinga multiple filter configuration, router 112 can conceal delay byreconfiguring one filter associated with the output link while anotherfilter associated with that same output link processes signals beingreceived.

FIG. 16 is a flow chart showing one example of a method 800 of reducingdelay by assigning a plurality of tunable transmitters to an input linkto the router. In this example, method 800 beings at step 810 where afirst tunable transmitter of a group of tunable transmitters associatedwith a single input to the router generates an optical router signalhaving primarily a first wavelength. Referring to FIG. 8 c for exemplarypurposes, while first transmitter 146 a 1 generates the optical routersignal having primarily the first wavelength, a second tunabletransmitter 146 an tunes to a second wavelength at step 820. In aparticular embodiment, second tunable transmitter 146 an substantiallycompletes tuning to the second wavelength before first tunabletransmitter 146 a 1 completes generation of the first optical routersignal. This process can be repeated at multiple groups of tunabletransmitters, each group associated with one input to router 112.

Router 112 communicates at step 830 a signal from each of the groups oftunable transmitters to star switching fabric 140. Star switching fabric140 communicates substantially similar sets of optical signals receivedto each of a plurality of filters. In this particular example, each ofthe filters comprises a fixed wavelength filter operable tosubstantially communicate a predetermined wavelength or range ofwavelengths and to reject other wavelengths. Each filter can beassociated with an output from router 112. Router 112 can facilitateselectively directing signals through switching fabric 140 byselectively tuning transmitters 146 to wavelengths of filters associatedwith desired output links from router 112. Like the method implementingmultiple tunable filters for each output link, using multiple tunabletransmitters for each input link conceals delay otherwise associatedwith reconfiguring tunable lasers of router 112.

In various embodiments, one or more switching time enhancing techniques,such as those described in FIGS. 12–16 can be combined to furtherincrease the switching time of the router.

Although various aspects of the present invention have been described inseveral embodiments, a myriad of changes, variations, alterations,transformations, and modifications may be suggested to one skilled inthe art, and it is intended that the present invention encompass suchchanges, variations, alterations, transformations, and modifications asfall within the spirit and scope of the appended claims.

1. A scheduler for use with a star switching fabric, the schedulercomprising: a scheduling star switching fabric operable to receive aplurality of packets each associated with one of a plurality ofwavelengths; a plurality of selecting elements associated with thescheduling star switching fabric, each operable to contribute toselectively passing packets from the scheduling star switching fabricfor receipt by a transmission star switching fabric, wherein packetsreceived at the transmission star switching fabric over a given timeperiod comprise a more uniform load distribution than packets receivedat an input to the scheduler over the same period of time; and anintermediate buffer stage residing between the scheduling star switchingfabric and the transmission star switching fabric, the intermediatebuffer stage operable to store packets received by the scheduling starswitching fabric pending transmission of those packets toward thetransmission star switching fabric, wherein the intermediate bufferstage is operable to store packets to reduce missequencing of packets atoutputs from the transmission star switching fabric.
 2. The scheduler ofclaim 1, wherein the scheduling star switching fabric comprises a signaldivider operable to receive a multiple wavelength signal and tocommunicate the multiple wavelength signal to a plurality of outputpaths from the scheduling star switching fabric.
 3. The scheduler ofclaim 2, wherein the signal divider comprises a cascade of 1×n opticalcouplers.
 4. The scheduler of claim 2, wherein the signal dividercomprises a power divider.
 5. The scheduler of claim 2, wherein thescheduling star switching fabric comprises a signal combiner operable tocombine a plurality of wavelength signals into the multiple wavelengthsignal and to communicate the multiple wavelength signal to the signaldivider.
 6. The scheduler of claim 2, wherein the signal divider iscoupled to an optical amplifier operable to amplify the multiplewavelength signal to at least partially compensate for a loss associatedwith the signal divider.
 7. The scheduler of claim 1, wherein theplurality of selecting elements comprise a plurality of tunable filters,each operable to receive a substantially similar set of packets from thescheduling star switching fabric and to selectively pass packets havinga selected wavelength.
 8. The scheduler of claim 7, wherein theplurality of tunable filters reside integrally to the scheduling starswitching fabric.
 9. The scheduler of claim 1, wherein the plurality ofselecting elements comprise a plurality of tunable optical transmitters,each operable to communicate to the scheduling star switching fabric apacket in an optical format comprising a selected wavelength.
 10. Thescheduler of claim 9, wherein the plurality of selecting elementscomprises a plurality of optical filters, each operable to receive asubstantially similar set of packets from the scheduling star switchingfabric and to pass toward the transmission star switching fabric packetshaving a particular wavelength.
 11. The scheduler of claim 1, furthercomprising a scheduling engine operable to generate control signals toinstruct the plurality of selecting elements as to which wavelength topass, wherein the scheduling engine communicates control signals to eachof the plurality of selecting elements in a round robin fashion.
 12. Thescheduler of claim 11, wherein the control signals received by any oneof the plurality of selecting elements comprises an instruction operableto cause the selecting element to select a different wavelength than alast wavelength processed by that selecting element.
 13. The schedulerof claim 11, wherein the scheduling engine communicates transmissioncontrol signals to a plurality of transmission selecting elementsassociated with the transmission star switching fabric, wherein thetransmission control signals instruct the plurality of transmissionselecting elements to tune to a selected wavelength in a round robinfashion.
 14. The scheduler of claim 1, further comprising an inputbuffer stage operable to store packets pending transmission toward thescheduling star switching fabric.
 15. A method of scheduling operationof a star switching fabric, comprising: receiving at a scheduler aplurality of packets each having a wavelength; communicating from ascheduling star switching fabric of the scheduler a plurality ofsubstantially similar sets of the plurality of packets; and selectivelypassing packets having selected wavelengths from the scheduling starswitching fabric for receipt by a transmission star switching fabric,wherein packets received at the transmission star switching fabric overa given time period comprise a more uniform load distribution thanpackets received at an input to the scheduler over the same time period,and wherein selectively passing packets having selected wavelengths fromthe scheduling star switching fabric to the transmission star switchingfabric comprises: receiving at a tunable optical filter associated withthe scheduling star switching fabric one of the substantially similarsets of the plurality of packets; tuning the tunable optical filter to aselected wavelength; passing from the tunable optical filter one of theplurality of packets having the selected wavelength; and communicatingin a round robin fashion a plurality of control signals each designatedfor a different one of the plurality of optical filters, wherein atleast some of the control signals instruct the receiving tunable opticalfilter to tune to a different wavelength than the last wavelengthprocessed by that filter.
 16. The method of claim 15, wherein each ofthe plurality of packets comprises a different wavelength.
 17. Themethod of claim 15, wherein selectively passing packets having selectedwavelengths from the scheduling star switching fabric to thetransmission star switching fabric comprises: tuning one of a pluralityof tunable optical transmitters to a selected wavelength; generating apacket at the selected wavelength using the one of the plurality oftunable optical transmitters; communicating the generated packet to thescheduling star switching fabric; and receiving the generated packet atone of a plurality of filters associated with the scheduling starswitching fabric, the one of the plurality of filters operable to passthe one of the selected wavelengths.
 18. The method of claim 17, furthercomprising: communicating in a round robin fashion a plurality ofcontrol signals each designated for a different one of the plurality oftunable optical transmitters; wherein at least one of the controlsignals instructs the receiving tunable optical transmitter to tune to adifferent wavelength than the last wavelength processed by thattransmitter.
 19. The method of claim 15, wherein the tunable opticalfilter resides within the star switching fabric.
 20. The method of claim15, wherein the tunable optical filter resides on a line card coupled tothe star switching fabric.
 21. The method of claim 15, furthercomprising: generating a plurality of transmission control signals, eachoperable to instruct one of a plurality of transmission selectingelements associated with the transmission star switching fabric to tuneto a selected wavelength; communicating the plurality of transmissioncontrol signals in a round robin fashion to the plurality oftransmission selecting elements.
 22. A network element operable todirect optical signals, the network element comprising: a scheduleroperable to receive in a given time period a plurality of packets eachassociated with a wavelength, the plurality of packets receivedcomprising a first load distribution, wherein the scheduler comprises: ascheduling star switching fabric operable to receive the plurality ofpackets and communicate a plurality of substantially similar sets of theplurality of packets received; a plurality of scheduler selectingelements associated with the scheduling star switching fabric, eachoperable to contribute to selectively passing packets from thescheduling star switching fabric for receipt by a transmission starswitching fabric; and a scheduling engine operable to generate controlsignals to instruct the plurality of scheduler selecting elements as towhich wavelength to pass, wherein the scheduling engine communicatescontrol signals to each of the plurality of scheduler selecting elementsin a round robin fashion; and a transmission star switching fabricoperable to receive the plurality of packets communicated from thescheduler and to communicate a plurality of substantially similar setsof the plurality of packets received to each of a plurality of tunabletransmission filters, each tunable transmission filter operable to passa particular packet toward an output link by tuning to a wavelengthassociated with that packet; wherein the scheduler is operable torearrange the order of packets communicated from the scheduler from theorder those packets were received so that packets received by thetransmission star switching fabric in the given time period comprise amore uniform load distribution than the first load distribution, andwherein the scheduler is operable to schedule tuning of the plurality oftunable transmission filters using a round robin algorithm.
 23. Thenetwork element of claim 22, wherein the scheduling star switchingfabric comprises a signal divider operable to receive a multiplewavelength signal and to communicate the multiple wavelength signal to aplurality of output paths from the scheduling star switching fabric. 24.The network element of claim 23, wherein the scheduling star switchingfabric comprises a signal combiner operable to combine a plurality ofwavelength signals into the multiple wavelength signal and tocommunicate the multiple wavelength signal to the signal divider. 25.The network element of claim 23, wherein the signal divider is coupledto an optical amplifier operable to amplify the multiple wavelengthsignal to at least partially compensate for a loss associated with thesignal divider.
 26. The network element of claim 22, wherein theplurality of scheduler selecting elements comprise a plurality oftunable filters, each operable to receive a substantially similar set ofpackets from the scheduling star switching fabric and to selectivelypass packets having a selected wavelength toward the transmission starswitching fabric.
 27. The network element of claim 22, wherein theplurality of selecting elements comprise a plurality of tunablescheduler transmitters, each operable to communicate to the schedulingstar switching fabric a packet in an optical format comprising aselected wavelength.
 28. The network element of claim 27, wherein theplurality of scheduler selecting elements comprise a plurality ofoptical filters, each operable to receive a substantially similar set ofpackets from the scheduling star switching fabric and to pass toward thetransmission star switching fabric packets having a particularwavelength.
 29. The network element of claim 22, wherein the controlsignals received by any one of the plurality of scheduler selectingelements comprises an instruction operable to cause the schedulerselecting element to select a different wavelength than a lastwavelength processed by that scheduler selecting element.
 30. Thenetwork element of claim 22, wherein the scheduling engine communicatestransmission control signals to the plurality of tunable transmissionfilters, wherein the transmission control signals instruct the pluralityof tunable transmission filters to tune to a selected wavelength in around robin fashion.
 31. A network element operable to direct opticalsignals, the network element comprising: a scheduler operable to receivein a given time period a plurality of optical packets associated with awavelength, the plurality of packets received comprising a first loaddistribution; a plurality of tunable optical transmitters each operableto receive a packet from the scheduler and to communicate the packet inan optical format having a selected wavelength; and a transmission starswitching fabric operable to receive a plurality of packets from theplurality of tunable optical transmitters and to communicate a pluralityof substantially similar sets of the plurality of packets received toeach of a plurality of transmission filters each operable to pass apacket having a particular wavelength toward an output link associatedwith that filter; wherein the scheduler is operable to rearrange theorder of packets communicated from the scheduler from the order thosepackets were received so that packets received by the transmission starswitching fabric in the given time period comprise a more uniform loaddistribution than the first load distribution, and wherein the scheduleris operable to schedule tuning of the plurality of tunable opticaltransmitters using a round robin algorithm.
 32. The network element ofclaim 31, wherein the scheduler comprises: a scheduling star switchingfabric operable to receive the plurality of packets and communicate aplurality of substantially similar sets of the plurality of packetsreceived; and a plurality of scheduler selecting elements associatedwith the scheduling star switching fabric, each operable to contributeto selectively passing packets from the scheduling star switching fabricfor receipt by a transmission star switching fabric.
 33. The networkelement of claim 32, wherein the plurality of scheduler selectingelements comprise a plurality of tunable filters, each operable toreceive a substantially similar set of packets from the scheduling starswitching fabric and to selectively pass packets having a selectedwavelength toward the transmission star switching fabric.
 34. Thenetwork element of claim 32, wherein the plurality of schedulerselecting elements comprise a plurality of tunable schedulertransmitters, each operable to communicate to the scheduling starswitching fabric a packet in an optical format comprising a selectedwavelength.
 35. The network element of claim 34, wherein the pluralityof scheduler selecting elements comprise a plurality of optical filters,each operable to receive a substantially similar set of packets from thescheduling star switching fabric and to pass toward the transmissionstar switching fabric packets having a particular wavelength.
 36. Thenetwork element of claim 32, wherein the scheduler comprises ascheduling engine operable to generate control signals to instruct theplurality of scheduler selecting elements as to which wavelength topass, wherein the scheduling engine communicates control signals to eachof the plurality of scheduler selecting elements in a round robinfashion.
 37. The network element of claim 36, wherein the controlsignals received by any one of the plurality of scheduler selectingelements comprises an instruction operable to cause the schedulerselecting element to select a different wavelength than a lastwavelength processed by that scheduler selecting element.
 38. Thenetwork element of claim 36, wherein the scheduling engine communicatestransmission control signals to the plurality of tunable opticaltransmitters associated with the transmission star switching fabric,wherein the transmission control signals instruct the plurality oftunable optical transmitters to tune to a selected wavelength in a roundrobin fashion.
 39. A method of scheduling operation of a star switchingfabric, comprising: receiving at a scheduler a plurality of packets eachhaving a wavelength; communicating from a scheduling star switchingfabric of the scheduler a plurality of substantially similar sets of theplurality of packets; and selectively passing packets having selectedwavelengths from the scheduling star switching fabric for receipt by atransmission star switching fabric, wherein packets received at thetransmission star switching fabric over a given time period comprise amore uniform load distribution than packets received at an input to thescheduler over the same time period, and wherein selectively passingpackets having selected wavelengths from the scheduling star switchingfabric to the transmission star switching fabric comprises: tuning oneof a plurality of tunable optical transmitters to a selected wavelength;generating a packet at the selected wavelength using the one of theplurality of tunable optical transmitters; communicating the generatedpacket to the scheduling star switching fabric; receiving the generatedpacket at one of a plurality of filters associated with the schedulingstar switching fabric, the one of the plurality of filters operable topass the one of the selected wavelengths; and communicating in a roundrobin fashion a plurality of control signals each designated for adifferent one of the plurality of tunable optical transmitters, whereinat least one of the control signals instructs the receiving tunableoptical transmitter to tune to a different wavelength than the lastwavelength processed by that transmitter.